1. Field of the Invention
The present invention relates to an optical disc, and more particularly, to device and method for reducing a jitter in an optical disc.
2. Discussion of the Related Art
In production of the optical disc, there are an injection molding process in which resin is injected into a mold to form an optical disc and a mastering process in which the mold for the injection molding is provided. In the mastering, a cutting process is included. And, in the cutting process, a process for directing a laser beam onto a glass coated with photoresist PR to etch the glass is included.
A background art optical disc device will be explained with reference to the attached drawings. FIG. 1 illustrates the background art optical disc device, schematically.
Referring to FIG. 1, the background art optical disc device is provided with a DVD formatter 1 for providing a DVD format signal in response to a DVD source signal for a digital versatile disc(called "DVD" hereafter), and a laser beam recorder(called "LBR" hereafter) for directing a laser beam modulated according to the DVD format signal from the DVD formatter 1 onto the DVD through different optical system to form pits in the DVD. The LBR 2 has an acoustic optic modulator(called "AOM" hereafter) 2a for switching the laser beam in response to the DVD format signal from the DVD formatter 1.
FIG. 2 illustrates a timing diagram in the DVD formatter shown in FIG. 1, FIG. 3 illustrates a difference of an ideal timing and an actual timing in the DVD formatter, FIG. 4 illustrates an ideal eye-pattern of FIG. 1, and FIG. 5 illustrates an asymmetry eye-pattern of FIG. 1. The background art optical disc device will be explained with reference to the attached drawings.
First, the DVD formatter 1 provides a DVD format signal in response to a received DVD source signal. That is, as shown in FIG. 2, the DVD formatter 1 provides a signal of 3T.about.14T at random in response to a DVD source signal. The 3T signal denotes a signal having a three time of frequency of a signal having a basic frequency T, 4T signal four times, and so on. Then, the LBR 2 modulates a laser beam according to the DVD format signal and directs the laser beam onto a DVD through different optical system to form pits therein. That is, the AOM 2a in the LBR 2 turns on/off the laser beam in response to the DVD format signal from the DVD formatter 1. As shown in FIG. 2, the laser beam turned on/off by the AOM 2a is directed onto the DVD through different optical systems, to form pits each corresponding to a number of times of the basic frequency. Though each of the pits is intended to have a length corresponding to a length of the DVD format signal from the DVD formatter 1, an actual length of the pit may have an error depending on characteristics of the LBR 2. This error affect much to a 3T signal with a short signal length and little to a 14T signal with a long signal length. That is, if signals from the pits in the optical disc is examined on an oscilloscope before any signal processing, centers of the signals fall on an axis as an ideal eye-pattern shown in FIG. 4. However, if the lengths of the pits are slightly longer, centers of the 3T signals deviate to a low side of the axis as an asymmetry eye-pattern shown in FIG. 5, which is called asymmetry in a signal analysis. That is, as shown in FIG. 3, when T.sub.w is assumed to be a frequency of a basic clock of the DVD signal, a frequency of an ideal 3T signal is 3T.sub.w. However, an asymmetry of 3T.sub.w +.DELTA.T.sub.w may occur of the real 3T signal in a read. In this instance, a jitter and a standard deviation may be expressed as the following equations (1) and (2). ##EQU1##
The jitter value is a percentage standard deviation of an actual 3T signal from an ideal 3T signal in terms of time period.
As there is no device for correcting the asymmetry occurred in the process of forming the pits in the background art, mastering conditions have been varied for correcting the asymmetry. However, the correction of an asymmetry by varying the mastering conditions has not been successful since there are too many variables in correction of asymmetry, such as an intensity of the laser beam, a thickness of the photoresist, a developing time period, a developing level, and the like, failing to match a pit length of a certain times.